unit returned isotopically reliable ages from 125.0 to 128.4
which is a significant age range considering the limited
thickness of the unit. This lack of stratigraphic consistency
indicates these ages may be suspect.
The expansion of sub-tropical reefs some 550 km past
the modern limit of reef development, clearly indicates
warmer sea temperatures than present during the LIG
(Kendrick et al., 1991). This expansion was examined in
detail by Greenstein and Pandolfi (2008), who contrasted
modern and Pleistocene coral-assemblage pairs from Cape
Range to Rottnest Island, a latitudinal gradient of 12
.
They found that in the north, between Cape Range and
Abrolhos, modern and fossil reefs had similar assemblages. But for sites further south, from Geraldton to
Rottnest, coral taxa were different and fossil reefs were
dominated by acroporas (A. palifera, A. humilis, and
A. hyacinthus). In other words LIG reefs at Rottnest Island
had a coral assemblage equivalent to the northern modern
reefs, rather than the present assemblage around the
island. This could be a preservational bias due to erosion
of the fossil reefs, but of the 31 fossil taxa, 8 had significantly expanded ranges. In addition, corals such as
A. palifera, which today is only found north of Cape
Range, occurred south of the Abrolhos during the LIG.
These results are clearly consistent with a shift in the biogeographic boundary of reef development and also
a reduced latitudinal temperature gradient during the LIG.
In terms of timing of the biogeographic shift,
Greenstein and Pandolfi (2008) assumed that the range
expansions of coral taxa were contemporaneous and that
reef-building events occurred during the same time interval. However, an analysis of the geochronology of reef
development at the same sites by McCulloch and Mortimer (2008) concluded that the expansion of these taxa
may have been confined to the early part of the LIG
between 129 and 125 ka. Such claims are clearly hinged
on the reliability of the
230 Th ages and, given that there
has been little assessment of the true-age variation in
corals at these sites it is difficult to rule out the possibility
that apparent age differences between the LIG reefs of
Western Australia are merely dating artifacts due to subtle
diagenetic processes, perhaps related to the increasing
aridity to the north.
From the foregoing, it is abundantly clear that there was
widespread reef development along the coast of Western
Australia during the LIG. Of the 16 sites described above,
12 have a reef terrace at or below þ3 m which is consistent
with an initial þ3 m SL position. Of the remaining four
sites, three have reef terraces between þ3 and þ4 m.
And the final site, at Cape Cuvier, shows evidence for
two sea-level positions, with a reef terrace at þ5 m and
a coralline-encrusted shore platform up to þ8 m.
Although it has been claimed that the Cape Cuvier site
represents evidence of a highstand greater than
þ6 m (O’Leary et al., 2008a), it is evident that the elevation of the reef and shore platform is anomalous in comparison with all other sites. This in itself is significant
evidence for neotectonic activity, but is also supported
by the site’s location on the western flank of the Cuvier
Dome (Logan, 1987). Interestingly, if we assume that the
reef terrace was uplifted from an original elevation of
þ3 m, then this implies that sea level must have subsequently risen to highstand of þ6 m at the end of the
LIG – a conclusion that is supported by elevations of SL
indicators from other parts of Australia (Murray-Wallace,
2002).
Stratigraphic analysis of the radiometric ages of corals
in the þ3 m reef terrace indicates that widespread reef
development along the Western Australian coast was confined to a much narrower time window than implied by the
simple spread of ages alone (Table 1). Clearly, the amount
of time represented by a 3 m interval of conformable reef
framework depends directly on the rate of reef accretion
and the precision of the age determination. During the last
deglaciation, for example, reefs accreted at rates of up to
3 m in 375 years during the rapid postglacial SL rise
(Blanchon, this volume). Measurement of the extrinsic
age variation in uplifted Holocene reefs on the Huon Peninsula shows that a 3 m stratigraphic interval took no more
than 1,000 years to develop (Edinger et al., 2007). Assuming the conservative estimate and an analytical error of
1,000 years, this indicates that the age of þ3 m reef terrace
along coastal areas of Western Australia should vary by no
more than 1,000 years, Æ 1,000 years (Blanchon et al.,
2009). As shown in Table 1, Yardie Creek in the north,
Burney and Leander Points in the centre, and Rottnest
Island in the south all have strictly reliable age ranges of
<2 ka and are therefore stratigraphically consistent. The
mean ages from these sites indicates a restricted age of reef
development between 120 and 124 ka for all but Rottnest
Island where the average age is 126 ka.
Hawaii
In addition to areas that have been largely stable since the
LIG, there are some areas that have undergone minor
amounts of gradual uplift thereby better exposing reef
development. For example, adjacent to the island of
Hawaii, where active volcanic loading has depressed the
lithosphere, lies the smaller island of Oahu. This island sits
upon the peripheral bulge of the loaded crust and, in contrast to the bigger island, has undergone slow uplift which
has preserved and elevated LIG deposits up to
þ13 m (Muhs et al., 2002). These deposits consist of inplace reef framework, overlain by erosively-based intertidal skeletal sand and gravel units, but very few have been
reported in any stratigraphic detail (Stearns, 1974; Ku
et al., 1974; Sherman et al., 1993; Szabo et al., 1994).
Recently Muhs et al. (2002) reported an extensive data
set of 37 high-precision
230
Th ages from this island and
claimed that the duration of the LIG extended from 136
to 115 ka and therefore predated the insolation peak as
claimed by Szabo et al. (1994) on the basis of less precise
ages. However, there are significant problems with this
analysis. First, Muhs et al. (2002) considered all ages with
initial ratios of 149 Æ 8 per mil as reliable, rather than
LAST INTERGLACIAL AND REEF DEVELOPMENT
633
which is a significant age range considering the limited
thickness of the unit. This lack of stratigraphic consistency
indicates these ages may be suspect.
The expansion of sub-tropical reefs some 550 km past
the modern limit of reef development, clearly indicates
warmer sea temperatures than present during the LIG
(Kendrick et al., 1991). This expansion was examined in
detail by Greenstein and Pandolfi (2008), who contrasted
modern and Pleistocene coral-assemblage pairs from Cape
Range to Rottnest Island, a latitudinal gradient of 12
.
They found that in the north, between Cape Range and
Abrolhos, modern and fossil reefs had similar assemblages. But for sites further south, from Geraldton to
Rottnest, coral taxa were different and fossil reefs were
dominated by acroporas (A. palifera, A. humilis, and
A. hyacinthus). In other words LIG reefs at Rottnest Island
had a coral assemblage equivalent to the northern modern
reefs, rather than the present assemblage around the
island. This could be a preservational bias due to erosion
of the fossil reefs, but of the 31 fossil taxa, 8 had significantly expanded ranges. In addition, corals such as
A. palifera, which today is only found north of Cape
Range, occurred south of the Abrolhos during the LIG.
These results are clearly consistent with a shift in the biogeographic boundary of reef development and also
a reduced latitudinal temperature gradient during the LIG.
In terms of timing of the biogeographic shift,
Greenstein and Pandolfi (2008) assumed that the range
expansions of coral taxa were contemporaneous and that
reef-building events occurred during the same time interval. However, an analysis of the geochronology of reef
development at the same sites by McCulloch and Mortimer (2008) concluded that the expansion of these taxa
may have been confined to the early part of the LIG
between 129 and 125 ka. Such claims are clearly hinged
on the reliability of the
230 Th ages and, given that there
has been little assessment of the true-age variation in
corals at these sites it is difficult to rule out the possibility
that apparent age differences between the LIG reefs of
Western Australia are merely dating artifacts due to subtle
diagenetic processes, perhaps related to the increasing
aridity to the north.
From the foregoing, it is abundantly clear that there was
widespread reef development along the coast of Western
Australia during the LIG. Of the 16 sites described above,
12 have a reef terrace at or below þ3 m which is consistent
with an initial þ3 m SL position. Of the remaining four
sites, three have reef terraces between þ3 and þ4 m.
And the final site, at Cape Cuvier, shows evidence for
two sea-level positions, with a reef terrace at þ5 m and
a coralline-encrusted shore platform up to þ8 m.
Although it has been claimed that the Cape Cuvier site
represents evidence of a highstand greater than
þ6 m (O’Leary et al., 2008a), it is evident that the elevation of the reef and shore platform is anomalous in comparison with all other sites. This in itself is significant
evidence for neotectonic activity, but is also supported
by the site’s location on the western flank of the Cuvier
Dome (Logan, 1987). Interestingly, if we assume that the
reef terrace was uplifted from an original elevation of
þ3 m, then this implies that sea level must have subsequently risen to highstand of þ6 m at the end of the
LIG – a conclusion that is supported by elevations of SL
indicators from other parts of Australia (Murray-Wallace,
2002).
Stratigraphic analysis of the radiometric ages of corals
in the þ3 m reef terrace indicates that widespread reef
development along the Western Australian coast was confined to a much narrower time window than implied by the
simple spread of ages alone (Table 1). Clearly, the amount
of time represented by a 3 m interval of conformable reef
framework depends directly on the rate of reef accretion
and the precision of the age determination. During the last
deglaciation, for example, reefs accreted at rates of up to
3 m in 375 years during the rapid postglacial SL rise
(Blanchon, this volume). Measurement of the extrinsic
age variation in uplifted Holocene reefs on the Huon Peninsula shows that a 3 m stratigraphic interval took no more
than 1,000 years to develop (Edinger et al., 2007). Assuming the conservative estimate and an analytical error of
1,000 years, this indicates that the age of þ3 m reef terrace
along coastal areas of Western Australia should vary by no
more than 1,000 years, Æ 1,000 years (Blanchon et al.,
2009). As shown in Table 1, Yardie Creek in the north,
Burney and Leander Points in the centre, and Rottnest
Island in the south all have strictly reliable age ranges of
<2 ka and are therefore stratigraphically consistent. The
mean ages from these sites indicates a restricted age of reef
development between 120 and 124 ka for all but Rottnest
Island where the average age is 126 ka.
Hawaii
In addition to areas that have been largely stable since the
LIG, there are some areas that have undergone minor
amounts of gradual uplift thereby better exposing reef
development. For example, adjacent to the island of
Hawaii, where active volcanic loading has depressed the
lithosphere, lies the smaller island of Oahu. This island sits
upon the peripheral bulge of the loaded crust and, in contrast to the bigger island, has undergone slow uplift which
has preserved and elevated LIG deposits up to
þ13 m (Muhs et al., 2002). These deposits consist of inplace reef framework, overlain by erosively-based intertidal skeletal sand and gravel units, but very few have been
reported in any stratigraphic detail (Stearns, 1974; Ku
et al., 1974; Sherman et al., 1993; Szabo et al., 1994).
Recently Muhs et al. (2002) reported an extensive data
set of 37 high-precision
230
Th ages from this island and
claimed that the duration of the LIG extended from 136
to 115 ka and therefore predated the insolation peak as
claimed by Szabo et al. (1994) on the basis of less precise
ages. However, there are significant problems with this
analysis. First, Muhs et al. (2002) considered all ages with
initial ratios of 149 Æ 8 per mil as reliable, rather than
LAST INTERGLACIAL AND REEF DEVELOPMENT
633